Multi-channel " stereo " modes sounding " louder " than discreet multi-channel modes is typical. What generally happens is that the SAME signal from a stereo source is pumped to ALL the speakers at the same level. The center is usually a combination of left and right and the back speakers will usually be mirrors of the left and right ( like the old quadraphonic. ) Of course HOW this is done depends upon the AVR brand. Surround modes don't sound as " loud " because the SUM TOTAL volume of all the speakers won't exceed the original signal volume by much. It is typically 70% for the Left, Center, Right, and remaining 30% for the surrounds. All this, of course, depends upon what is done in the mixing room. A lot of older stuff that was originally in Stereo and " up-mixed " to surround will actually sound " fuller " because there really wasn't any discreet surround data originally and it either has to be taken from the stereo mix or added in post-production, whether it is through additional foley work or digital sound effects.
I generally play everything in Audyssey DSX and switch to DTS Neo-X or PLIIz when something doesn't sound right. I rarely ever use the Multi-Channel Stereo mode now.
To me it is the opposite now, but I have PLENTY of power to go around. Playing something in 2-channel mode sounds flat and boring, even in 5/7/9-channel Stereo mode. Using Audyssey provides a little depth to the soundtrack/music but that may be because I'm not using any room treatments. These may be imperfections in my untreated room and would probably make the purists cringe. LOL
HD-DVD = 94
Blu-Ray = 120 ( 24 Warner red2blu )